When I know which ports are involved I shall be able to give some sort of sensible answer, well, as sensible as I can make it.
In general NZ is a huge mixtue. Cities tend to be very small, (the population is not much more than 3 million, but very diverse. Parts of NZ are quite spectacular, mainly on the South Island, where you find a lot of mountainous scenery which stretches down to Fjords such as Milford Sound. If you are looking for razamataz NZ is not the place. Most ship visits are to the North island. Auckalnd, Bay of Islands etc. Auckland is an administration city so not very touristy. The Bay of Islands, at the very Northern Tip of NZ has a slight sub-tropical atmosphere about it and would the closest that NZ has to offer if compared to parts of The Australian Whitsundays or the Caribbean. If the ship also stops at Tauraga in the Bay of Plenty you do have time to visit the thermal region of Rotorua, which is interesting, if a bit smelly from the sulphur fumes. You always have to remember that NZ is called 'The Land of the Long White Cloud' and it is well deserved - at any time of the year make sure you have a raincoat close at hand.
Australia is as different from NZ as Canada is to Caribbean. Most ships call to the East coast ports Sydney, Brisbane, Cid Harbour (for the Whitsundays) & Cairns. Sydney most people know quite well so I don't really have to say anything. Brisbane is the capital of Queensland and is again an administration city although there are some good local trips to take, or you can get to the Gold Coast in around an hour which is probably the best option. The Gold Coast is Australia's seventh largest city and devoted to only one purpose - making travellers welcome and happy. The hundreds of kilometres of Golden beach help things along.
If you go to an anchorage in the Whitsundays a whole new world opens up. When you go out on deck in the morning you will find your ship surrounded by beautiful islands, luxury yachts and catamarans rushing around and an atmosphere that seems to say - OK so slow down for a bit this is a place where you just let things happen. On the mainland and the islands you will see how green and tropical this wonderous part of the world can be. Visit one or two of the numerous islands during the day and you will see why this region is called the world's best kept tourist secret. If you wish you can book on one of the big cats out to the Great Barrier Reef - I would be surprised if the ship hadn't at least one tour doing this. However, unless you want to do some SCUBA or snorkelling you aren't going to see much, as all the good stuff is underwater and it's not quite the same from a glass bottom boat - still it is a lot of fun just going for the ride.
Cairns is a northern city that sits and swelters in the sun. very tropical with great marine and rainforest tours. The harbour is constantly being upgraded to keep up with passenger ship requirements but I would be very surprised if and ships like the 'Grand' class could make it in. To me Cairns is the sort of place whre I would take a short tour to get to see a bit of the area, then spend the rest of the day just mooching around town. All the Australian regions you will find clean, well serviced, with safe food and water and quite a friendly mob of people you help you along.
Shopping - NZ - Wool items - small carvings
Australia - Just about everything from opals to aboriginal art.
Pricing- New Zealand - can be a bit overpriced on food items but if you stay away from the obvious traps - quite reasonable on other things.
Australia - Sydney - gets a bit expensive as they get the bulk of the tourists - so mark up accordingly. - Brisbane cheaper - good range - but you have to go looking for it. The Whitsundays - Can get expensive as most things have to be flown or shipped in - Seafood splendid at at very reasonable prices. This region really into the t-shirt sort of thing.
Cairns - Similar to Whitsundays - slightly better range - and you can finds some good bargains if you go looking. Restaurants - unless you go looking for five star it is quite reasonable and you do get to try some dishes that you probably haven't tried before.
One good thing - If you have US$ the current rate of exchange means that you almost double your spending power - and the cost of things is about the same as at home - so you can afford a little more fun